154 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Note. Carbon monoxide is obtained when anhydrous potassium ferro- 

 cyanide is heated with concentrated sulphuric acid. 



Hydrogen cyanide is also obtained by dehydrating ammonium 

 formate, or formamide, with phosphorus pentoxide. 



Pure anhydrous hydrogen cyanide is prepared by distilling potas- 

 sium cyanide with moderately concentrated sulphuric acid, passing 

 the gas over anhydrous calcium chloride and collecting the distillate 

 in a receiver cooled by ice. 



Properties. 



Pure hydrogen cyanide is a colourless, mobile liquid having a 

 specific gravity of "697 at 18. It becomes a crystalline solid at - 15 

 and it boils at 26' 5. It has a peculiar smell, resembling that of oil 

 of bitter almonds and is intensely poisonous. It burns with a violet 

 flame and is easily soluble in water and alcohol. 



It is a very weak acid and turns blue litmus only a faint red 

 x Reactions. 



Aqueous solutions of hydrogen cyanide are unstable and slowly 

 undergo decomposition into ammonium formate : 



HCN + 2H 2 O = HCOONH 4 . 



The pure acid is also rapidly decomposed by concentrated hydro- 

 chloric acid. Formamide is first formed and this passes into formic 

 acid and ammonium chloride : 



HCN + H 2 = HCONH 2 

 HCONH 2 + HC1 + H 2 O = HCOOH + NH 4 C1. 



Hydrogen cyanide is thus the nitrile of formic acid. 



The salts of hydrocyanic acid are decomposed in the same way on 

 boiling their aqueous solutions : 



If about 20 c.c. of a i per cent, potassium cyanide solution be 

 boiled for some time it is converted into ammonia and potassium 

 formate. The ammonia is readily detected by its action on reel 

 litmus paper and the formate may be detected by testing the solution, 

 after the ammonia has been given off, with (i) ferric chloride and with 

 (2) mercuric chloride, as on p. 95. 



Hydrogen cyanide in alcoholic solution is reduced by sodium to 

 methylamine : 



HCN + 2 H 



